<!doctype html>
<head>
  
<body>
  <canvas id="mycanvas" width="200" height="600"></canvas>

  <script type="text/javascript">
  var cx = document.getElementById('mycanvas').getContext('2d');
  cx.beginPath();
  cx.rect(10, 10, 50, 50);
  cx.clip();
  cx.beginPath();
  cx.rect(0, 0, 50, 50);
  cx.shadowColor = "black";
  cx.shadowOffsetX = 10;
  cx.shadowOffsetY = 10;
  cx.fill();
  </script>
